This invention relates generally to electrical discharge machining and more particularly, to an electrical discharge machine having more than one electrode.
Conventional machines for electrical discharge machining (EDM) have a single electrode carried by a holder, movable relative to a metallic workpiece by a servo-controlled actuator and energized by a pulsed electrical current to form holes. slots or cavities in the metallic workpiece. An operative end of the electrode is spaced from the workpiece providing a spark gap therebetween which permits a spark discharge to pass between the electrode and the workpiece. Each spark produces heat sufficient to melt or vaporize the adjacent portion of the workpiece leaving a pit or cavity in the workpiece. EDM is ideal for applications involving workpieces that are difficult to machine, have a complex shape, require small slots, holes or passages and close tolerances.
The single electrode of a conventional EDM machine enables only one hole size to be formed into a single workpiece. To form different sized holes in a workpiece, the machine must be taken out of production for a tooling change which greatly increases the down time of the system and significantly increases the time and cost to manufacture a workpiece. Still further, the operator of the machine may not be able to change the tooling requiring another individual to perform this operation which extends the down time and further increases the cost to manufacture the workpiece. Alternatively, the workpiece may be carried on a fixture to multiple workstations, each having a separate EDM machine, to form different sized holes in the workpiece. This significantly increases the complexity and cost to manufacture a workpiece because multiple EDM machines are needed, and the fixture and transport mechanisms must be extremely accurate and reliable to form consistent workpieces. Introducing multiple EDM machines, fixtures and a transport system significantly reduces the reliability of the system as a whole thereby increasing the system down time and increasing the cost of manufacture.
An EDM machine with a plurality of separate electrodes on the same head of the EDM machine and selectively driven by the same actuator, is provided. Preferably, all of the electrodes are not used to cut the workpiece at the same time. Rather, the actuator has a drive mechanism, which is preferably servo-controlled, and each electrode is selectively engaged with the drive mechanism to advance and retract only the desired electrode relative to the workpiece. Either the workpiece or the EDM head may be manipulated using a CNC positioning system to accurately locate the desired electrode relative to the workpiece.
The EDM machine preferably has a carriage driven between retracted and advanced positions relative to a base by the drive mechanism. Each electrode is selectively coupled to the carriage for co-movement therewith between corresponding retracted and advanced positions, by a separate drive clamp. To advance or retract one of the electrodes, the drive clamp is closed and the carriage is moved relative to the base to move the electrode with the carriage. When the drive clamp of an electrode is open, the electrode is not coupled to the carriage so that the carriage may be moved relative to the electrode which remains in its retracted position when it is not needed.
To account for wear of the electrodes and to reset their position relative to the carriage, a re-feed clamp carried by the base may be provided for each electrode. As the electrode wears away, it can be replenished or reset by advancing the remaining electrode relative to the carriage. To do this, on the retract stroke of the carriage the re-feed clamp of the electrode is closed to clamp the electrode and the drive clamp of that electrode is opened, releasing the electrode from the carriage. Continued movement of the carriage to its retracted position and relative to the electrode will, in effect, advance the electrode relative to the carriage to reset the position of the operative end of the electrode relative to or pull the electrode further out of the carriage. Each re-feed clamp may also be used to maintain its corresponding electrode in its retracted position when the carriage is advanced and the electrode is not needed.
According to one aspect of the invention, each electrode may be of a different size or shape to permit differently sized holes, slots or cavities and the like to be formed in a workpiece at the same workstation in a single fixture. Another object and advantage of the invention is that a wide range of holes, slots or cavities may be formed with a single EDM machine using a single head driven by a single drive actuator, typically servo-controlled, and without having to change tooling or provide multiple EDM heads, multiple drives and controls therefor, or multiple workstations and a system to transport the workpieces between workstations. Yet another advantage of the invention is that multiple workpieces requiring different holes, slots and the like may be machined by a single EDM without any tooling change.